Masdevallia menotoi and M. pyxis
 

I'm having trouble determining Masdevallia menotoi from Masdevallia pyxis. What are the keys to correctly identify them?

I always go to the Internet Orchid Species Photo Encyclopedia, orchidspecies.com .

If you look at the photos, you won't be able to see any major differences.
On orchidspecies, if you look at the two photos of Masdevallia menotoi, they appear to be two different plants.